Owen Daly, as I recall, is partial to using Tru-Oil to finish boxwood keytops on his Italian harpsichords. Tru-Oil contains polymerized linseed oil among other things and is made for varnishing rifle stocks, but Owen and others report excellent outcomes using it on keyboards. Although I don't recall Owen explicitly saying he uses it on ebony accidentals, it seems odd to me he'd use it on the naturals but not on the sharps.
I have used Circa 1850 Antique Danish Oil on a boxwood and ebony Italian keyboard I made myself. "Danish oil" implies an oil-varnish blend. You get the penetrating qualities of oil with some of the film-building of a varnish, but not too much of either. It has held up well for almost a decade, so the product seems good to me. I would closely follow David J.'s advice: apply sparingly, allow to penetrate briefly, then THOROUGHLY wipe off all the excess and keep wiping for some time after the first wipe, in case more oozes out. If you don't remove anything that bubbles back out it will lead to a sticky disaster when it cures.
As an aside, there is a great deal of nonsense labelling in the world of wood finishes. Although the product above is a true Danish oil (it mentions the oil-varnish blend right on the label), there are similarly-named products that don't in any way contain an oil-varnish blend. Likewise you can buy "tung oil" and "teak oil" at places like Home Depot that contain neither of those ingredients. It seems the manufacturers base the name on the cosmetic results you are likely to get with their product, and sometimes it is up to you to figure out what you're actually buying. So be careful and read those labels.
BM
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